Methods of cascading semiconductor die have been in use in the semiconductor industry since the 1990s. This technology has been primarily used for memory chips in computer systems. With increasing trends towards systems on chips this packaging methodology is rapidly expanding into other applications. Stacking of semiconductor die is ideal for low current, low power systems. Thermal considerations have prevented these packaging methods from being used for higher power devices.
Typical IGBT (Insulated Gate Bipolar Transistor) module die arrangements have the dies attached to copper which is bonded to a ceramic backplate. The heat generated by the dies is transferred through the ceramic to the heat sink located under the backplate. Bonded wires are used for electrical connection between the dies and electrodes. The connection wires are encased in a podding gel for insulation, thermal transfer, and mechanical stability purposes. Thermal resistance along the heat transfer path makes the junction temperature higher than it would be for a directly liquid cooled arrangement.
With dies stacked, the small die resides atop the larger and wire bonds are brought out to either the substrate or a lower level die. This method can be expanded to larger numbers of layers. Using several layers of die, the topmost layer has two separate dies sitting on the layer beneath it. Cascading die in this manner results in increases in packing density, reductions in cost, less inductance, and faster signal transmissions because the dies are closely stacked. Most stacking methods still rely on wire bonds for bringing the I/O from the outside world to the silicon.
Within the semiconductor packaging industry there is a growing desire to move away from the wire bonding of the die to the substrate or output pins. Size, performance, and cost considerations are driving new packaging methods. This is true for low as well as high power devices.
Silicon Power Corporation is developing a new packaging method for power devices. The wire-bondless package is a soldered assembly of a semiconductor power device, such as an IGBT, and a thin ceramic lid. The lid is metalized on the bottom side and designed to mate to large and small device electrodes, which are connected by metalized vias to a more rugged and convenient pattern of top-side metal. If the lid material is a good conductor and/or if the lid vias are very dense, the ThinPak lidded device can be cooled from both sides or treated as a flip chip device, but without the usual limitations in achievable breakdown voltage. The low impedance and small size and weight of the device, as well as the rather large mechanical tolerances of the lid, make it convenient for module applications.
The thin-film power overlay (POL) technology developed at General Electric is an approach to reducing the cost of mass production and improving the reliability and efficiency of power electronics packages. Power semiconductor devices are soldered to a Direct Bonded Copper (DBC) substrate from the backside. Differences in device thickness are compensated by copper shims. A thin layer of polyamide sheet is laminated over the die after vias are laser machined or mechanically punched through the film. These vias provide openings for the power interconnect to the top layer. The whole top surface is then metallized (electro-plated) with copper. Circuit patterns are achieved by the application of photo resist and chemical etching processes. More layers can be built up repeatedly to realize a multilayered interconnect structure. Low-profile passive components can be embedded into the overlay flex.
Dimple Array Interconnect (DAI) packaging involves the use of a copper sheet with arrays of dimples preformed serving as electrical interconnections. The Dimple Array Interconnect structure has similar shape as the hourglass-shaped flip chip interconnect.
International Rectifier's DirectFET is a surface-mount package that improves MOSFET performance by lowering both the package's electrical and thermal resistance. It does so with a design that permits direct attachment of the die to the pc board via solderable pads on the chip and through attachment to a copper drain clip that allows double-sided cooling. The DirectFET package consists of a passivated die attached to a copper clip. Solderable metal contacts on the bottom of the die provide gate and source contacts to the pc board, while the copper clip provides an electrical connection to the drain and permits topside cooling. Although the package has the same outline as an SO-8, the DirectFET's height is 60% less.
In Vishay Siliconix's PowerConnect technology, traditional bondwires are replaced with a direct connection between the MOSFET die and the copper leadframe to lower a package's contribution to the device's on resistance, RDS(on) in low-voltage power MOSFETs. To accomplish this direct connection, the top surface of the MOSFET die had to be made solderable. Toward that end, the company developed a nickel-based metallization process on top of the aluminum. The result is that the leadframe can be attached to both the bottom and the top surface of the die.
Semikron is making baseless IGBT power modules based on pressure contact packaging technology through the use of a spring pad and a pressure plate. The elimination of the base plate and thus the solder joint between the base plate and substrate, together with the use of spring contacts to establish connections between the built-in gate drive board and the substrate, leads to improved reliability and enables a very cost effective module and power electronic system assembly. Semikron packaging is designed for traditional heat-sink cooling and is not applicable to direct refrigerant cooling. The chips are cooled by a heat sink.
Virginia Polytechnic Institute and State University disclosed a three-dimensional packaging technique developed for power electronics building blocks using direct copper bonding to interconnect power devices. The parallel-plate structure provides the potential for double-sided cooling, direct liquid cooling of the power devices between the plates, and integration of passive components in the module.
FlipChip technology uses a ball grid array package resulting in a wire bondless system. The interconnection between the die and carrier in flip chip packaging is made through a conductive “bump” that is placed directly on the die surface. The bumped die is then “flipped over” and placed face down, with the bumps connecting to the carrier directly. After the die is soldered, underfill is added between the die and the substrate. Underfill is a specially engineered epoxy that fills the area between the die and the carrier, surrounding the solder bumps. It is designed to control the stress in the solder joints caused by the difference in thermal expansion between the silicon die and the carrier. Once cured, the underfill absorbs the stress, reducing the strain on the solder bumps, greatly increasing the life of the finished package. The chip attach and underfill steps are the basics of flip chip interconnect.
A flex-circuit interconnection system is being developed at the Center for Power Electronics Systems at Rensselaer Polytechnic Institute. This method offers extra layout design freedom in the vertical dimension of the package. Compared with the conventional power module, the power terminals in the flex-circuit package are distributed on the DBC and the flex substrate with close proximity and planar configuration. As a result, the package parasitic inductance can be reduced significantly.